1. Field of the Invention
The present invention relates to a method of producing a printer head of an on-demand type, which is suitable for use in a so-called ink-jet type printer for printing liquid ink onto a sheet of paper.
2. Description of the Related Art
As printer heads of so-called on-demand types for jetting ink drops in accordance with painting instructions, there are known one of a type for bubbling ink with heat and jetting it as has been disclosed in Japanese Patent Publication No. 61-59913, for example, and one of a type for applying an electric field to a piezoelectric element and jetting ink owing to the deformation of the piezoelectric element as has been described in Japanese Patent Application Laid-Open Publication No. 55-11811.
According to the former (the invention disclosed in Japanese Patent Publication No. 61-59913 or the like), respective jetting units are reduced in size. Therefore, a number of nozzles can be arranged at high density. However, this disclosure is accompanied by drawbacks that bubbling an optical density of the ink cannot be increased because of the bubbling and the ink is burned in a heating plate for heating the ink, thereby impairing durability of the jetting units. According to the latter (the invention disclosed in Japanese Laid-Open Patent (Kokai) No. 55-11811 or the like), problems concerning to the optical density of the ink and the durability of the jetting units do not arise. Since, however, the width of a piezoelectric element increases, a number of nozzles cannot be arranged at high density.
FIG. 8 shows the printer head using the piezoelectric element, which has been disclosed in Japanese Laid-Open Patent (Kokai) No. 55-11811, for example. As shown in FIG. 8(a), a liquid reservoir 21, a plurality of pressure chambers 22 connected to the liquid reservoir 21 and having a diameter of about 2 mm, and a plurality of channels 23 coupled to the pressure chambers 22 are formed in a substrate 20 by etching. The channels 23 are gradually narrowed toward the tips of nozzles 24. As shown in FIG. 8(b), other substrate 25 has a plurality of piezoelectric elements 26 arranged in corresponding relationship to the pressure chambers 22. A desired printer head is formed by stacking the substrates 20 and 25 on each other and joining them. In this printer head, the voltage is applied to a desired piezoelectric element 26. Ink drops are jetted from the nozzle 24 owing to a variation in the capacity of each pressure chamber 22 based on the deformation of the piezoelectric element 26.
However, the printer head shown in FIG. 8 causes pressure losses when the pressure developed in each pressure chamber 22 is transmitted to each channel 23. The pressure losses differ in magnitude or level according to the size of each channel 23 and thus jetting characteristics of ink from a plurality of nozzles 24 also differ from one another. This tendency often appears with an increase in the number of the nozzles 24 and hence the number of the nozzles 24 cannot be increased.
There are also known printer heads wherein piezoelectric elements are used and a number of nozzles are provided as has been disclosed in Japanese Laid-Open Patent (Kokai) No. 63-252750 and Japanese Laid-Open Patent (Kokai) No. 2-150355. A description will now be made of the invention disclosed in Japanese Laid-open Patent (Kokai) No. 2-150355 with reference to FIG. 9. The bottom sheet 30 is polarized in the direction indicated by the arrow, and includes a number of parallel grooves 31 defined by side walls 32 and a bottom surface 33. Further, the top opening surfaces of the respective grooves 31 are closed by joining a top sheet 35 to the top 34 of each side wall 32. Metal electrodes 37 are formed under evaporation on the internal surfaces, corresponding to both internal surfaces of the respective grooves 31, of the side walls 32 so as to fall within a range of about one-half the entire height of each groove as seen on the top sheet 35 side.
That is, the bottom sheet 30 is held by a jig in a vacuum deposition device. Then, a parallel beam of deposition metallic atoms is induced toward the bottom sheet 30 with an angle of .delta. formed with respect to each side wall 32 as shown in FIG. 10. Thus, a metallic film is deposited on a portion of one surface of each side wall 32. Then, the parallel beam of deposition metallic atoms is introduced into the bottom sheet 30 in the same manner as described above in a state in which the bottom sheet 30 has been turned 180.degree. with respect to the horizontal direction in FIG. 10. Thus, the metallic electrodes 37 are deposited on a range equal to about one-half the upper portion of both side surface of each side wall 32. At this time, the metallic film deposited on the top 34 of each side wall 32 is removed in the successive step.
Further, each of the pressure chambers is defined by closing each groove 31 with the top sheet 35. Thereafter, supply ports, which are in communication with an ink supply unit, are defined in one ends of the pressure chambers and jetting ports for jetting ink are defined in the other ends of the pressure chambers, thereby completing a printer head.
In this type of printer head, when voltages opposite in polarity to each other are applied to the electrodes 37 of the adjacent two side walls 32, the side walls 32 are subjected to the potential in the direction orthogonal to the polarity indicated by the arrow, of the bottom sheet 30, thereby producing shearing strain as indicated by the dot lines in FIG. 9. As a consequence, the capacity of the pressure chamber (groove 31) between the side walls 32 which has produced the shearing strain is abruptly reduced to increase pressure in the pressure chamber, thereby jetting ink from the jetting ports.
In the printer head disclosed in Japanese Laid-Open Patent (Kokai) No. 2-150355, as shown in FIGS. 9 and 10, about eight nozzles (jetting ports) can be arranged at high density within a range of width of 1 mm. Further, a pressure loss is not produced between each pressure chamber and each nozzle. Thus, an increase in the number of the nozzles can be effected. However, the printer head has the following problems.
A first problem is that the manufacturing cost becomes high because a method of forming electrodes is cumbersome and the electrodes 37 are formed by using an expensive vacuum deposition device.
A second problem is that an uniform electric field cannot be applied across the bottom sheet 30 formed of a piezoelectric material. That is, since the piezoelectric material is normally of a calcined member formed of crystalline particles, grinding surfaces produced by forming each groove 31 are of grinding surfaces having irregularities developed as the crystalline particles are. On the other hand, the metallic deposition using the vacuum deposition device for forming the electrodes 37 is not effected for portions not opposite to a deposition metallic atoms emitting source. Accordingly, the metal is deposited only on each convex portion on the surface of the grinding surfaces of the grooves 31 and is not deposited on the concave portion. Each concave portion serves as a pinhole. Therefore, the uniform electric field cannot be applied to the bottom sheet 30.
A third problem is that it is necessary to form protection films because the grinding surfaces of the grooves 31 are corroded by being in contact with the ink and the protection films is hard to form. Since the bottom sheet 30 is formed of the piezoelectric material, it has concave-convex surfaces. It is therefore so difficult to form protection films comprised of Si.sub.3 N.sub.4 or SiON so as to avoid the pinholes. Further, since the above-described electrodes 37 also have pinholes, they cannot be functionally anticipated as being the protection films.